1. Introduction
This invention relates to a step of catalysis in a process for metal deposition and more particularly, to an improved accelerator composition for use in combination with a tin containing electroless metal plating catalyst for improved deposition properties.
2. Description of the Prior Art
Metal deposition over a substrate may be by electroless deposition, electrolytic deposition or a combination of the two. Electroless deposition is the chemical deposition of a metal or mixture of metals over a catalytic surface by chemical reduction and processes for electroless metal deposition are disclosed in U.S. Pat. Nos. 2,702,253 and 3,011,920 incorporated herein by reference. If the substrate to be metal plated is inert--i.e., not catalytic to metal deposition, the conventional process of plating comprises pretreatment to promote cleanliness and adhesion, catalysis of the substrate prior to deposition by treatment with a suitable plating catalyst that renders the surface catalytic to electroless metal deposition followed by a step identified by the art as acceleration. Plating catalysts are disclosed in the aforesaid patents.
Electrolytic deposition is the deposition of a metal over an electrically conductive substrate where a part to be plated serves as one of the electrodes in an electrolytic cell. A recent process for electrolytic deposition of a non conducting substrate is disclosed in U.S. Pat. Nos. 3,099,608 and 4,895,739, both incorporated herein by reference wherein an inert substrate is made sufficiently conductive for direct electroplating by a process using the same type of plating catalyst as the electroless plating process described above. Following catalysis with a catalyst of the type disclosed in the aforesaid U.S. Pat. No. 3,011,920, a part is treated with an accelerator and electrolytically plated without an intermediate electroless plating step.
The catalyst most in commercial use for each of the above electroless and electrolytic plating processes comprises the reaction product of a substantial molar excess of stannous tin with palladium ions in hydrochloric acid solution. The reaction product is believed to be a tin palladium colloid. It is believed that the oxidized stannic tin in combination with unreacted stannous tin and palladium ions form a protective, possibly polymeric, complex for the palladium or palladium-tin alloy while the unreacted stannous ions act as an antioxidant. Colloidal tin-palladium catalysts were first described in U.S. Pat. No. 3,011,920 incorporated herein by reference.
An improvement in colloidal tin palladium catalysis is disclosed in U.S. Pat. No. 3,904,792 incorporated herein by reference. In this patent, to provide a catalyst that is less acidic than those disclosed in the aforesaid U.S. Pat. No. 3,011,920, a portion of the hydrochloric acid is replaced by a solution soluble metal halide salt of the acid resulting in a more stable catalyst having a pH that can approach about 3.5. The catalysts of this patent are in significant commercial use.
It is known in the art that in use of a catalyst formed from the reaction product of stannous tin and noble metal ions, a process sequence would typically include the steps of catalysis of the substrate, acceleration of the catalytic layer, typically with an acid such as fluroboric or perchloric acid and electroless or electrolytic metal deposition. The step of acceleration is known to activate the palladium catalyst, enhance the initiation of the plating reaction and decrease the plating time for total coverage of the part to be plated. Though much has been written about the step of acceleration, the function of the accelerator is still not fully understood. The prevailing explanation in the art is that the accelerator dissolves both unreacted stannous salt and stannic acid surrounding the catalytic noble metal particle adsorbed onto the surface of the part to be plated thus exposing them and permitting the noble metal to function as a catalyst. A lessor known but plausible theory is that the acid environment causes autoreduction of a tin noble metal complex surrounding the noble metal particle on the surface of the part to be plated. Regardless of the theory, it is known that the step of acceleration significantly improves the efficiency of the plating reaction and the quality of the metal deposit.